The present invention relates to a sheathed-element glow plug to be mounted in a combustion chamber.
Sheathed-element glow plugs, having a metallic housing, to be mounted in a combustion chamber are already known. A rod-shaped heating element is situated in a concentric bore hole of the known sheathed-element glow plug, the heating element having a first current-carrying layer and a second current-carrying layer, the cross sections of the first and the second current-carrying layer being connected on the end of the heating element on the combustion chamber side via a conducting-layer crosspiece. In this context, the first and the second current-carrying layers are separated by an insulating layer. Furthermore, sheathed-element glow plugs are known whose current-carrying layers vary in length.
In contrast, the sheathed-element glow plug of the present invention has the advantage that there is no danger of a short circuit at the end of the heating element away from the combustion chamber. A further advantage is that the contact surface between the first current-carrying layer and the contact element situated at the end of the heating element away from the combustion chamber is enlarged. The contact resistance is consequently reduced, thereby resulting in the contact point heating up less. Therefore, the danger of the contact material between the heating element and the contact element being thermally destroyed is decreased. In addition, it is advantageous that the rod-shaped heating element does not need to be adjusted to remove an insulating layer situated on the rod-shaped heating element in the region in which the current supply is to be contacted.
It is particularly advantageous to also design the insulating layer to be asymmetrical, so that the danger of a short circuit due to damage or a porousness of an insulating layer deposited on the heating element is also decreased in this instance. In this context, it is advantageous to expand the region in which the insulating layer is asymmetrically formed in the direction of the combustion chamber over the collar of the heating element, since a summation of form-dependent and material-dependent stress concentration is prevented in this manner. In addition, it is advantageous to design the heating element such that an advantageously half-shell-shaped insulating layer made of an electrically insulating, ceramic material is deposited in the region in which the first current-carrying layer extends into the housing, the insulating layer between first and second current-carrying layer being made of the same material. As a result, the manufacturing process is simplified and, thus, more cost-effective. In order to be able to dispense with additional insulation, it is advantageous to design the insulating layer such that it extends beyond the end of the housing on the combustion chamber side. Furthermore, it is advantageous to provide a stepped lug at the end of the heating element away from the combustion chamber, so that an adapter sleeve situated on the end of the heating element away from the combustion chamber and the contact element are able to be easily positioned.